168 EXPERIMENTAL PHYSIOLOGY. 



the brightest, but most normal individuals if shown a spectrum in a brightly 

 lighted room, pick out the yellow as the brightest part of it, brighter than either 

 red, orange, green, blue or violet. If however all light is shut out of the room 

 except that coming through the prism which makes the spectrum, and if the eyes 

 are allowed to become adapted to the dark, the place of greatest brightness in the 

 spectrum appears to shift from yellow to green. This colour, it is to be noted, is 

 also the most effective in bleaching the visual purple. The spectrum under these 

 conditions also appears shortened at the red end, that is to say the long waves are 

 ineffective for the eyes in this state, as they are also found to be on the purple 

 itself. 



Some further evidence which bears on the same question is got from obser- 

 vations on the ACHROMATIC (or colourless) THRESHOLD. If, after the eyes have 

 become adapted to a dark room, light is gradually introduced, all objects in the 

 room of any colour except red look colourless when first they become perceptible, 

 as if they were made up of varying shades of white and grey, and it is only on 

 farther illumination that they appear in their own tints. The interval during 

 which they seem uncoloured is the achromatic interval. Red objects have no 

 such interval; they appear at once in their own colour, but they require a stronger 

 light to make them visible than is needed to make the other objects show as 

 colourless grey ones. The whole phenomenon can be seen out of doors at dawn, 

 or, in the reverse order, at dusk. At the first daylight everything comes into 

 view as if in a print, all black, white or grey, and it is only as the light increases 

 that the leaves change from grey to green, blues, yellows and violets appear, and 

 red flowers turn from black objects to brightly coloured ones. These results are 

 interpreted as follows : in the light the cones have a threshold lower than that of the 

 rods and, since they are capable of distinguishing colours as well as merely bright- 

 ness, we see all light in which there is more of some wave lengths than of others as 

 coloured light, no matter how faint it may be. In the dark adapted eye the 

 threshold of the cones is not changed, but that of the rods is lowered, probably 

 because of the presence in them of the visual purple, so that they become more 

 sensitive than the cones. The rods are not organs of colour vision, however, 

 when stimulated they give rise only to sensations of brightness. Therefore faint 

 lights of wave lengths other than red are first seen by the rods of the dark-adapted 

 eye as colourless rays and only affect the cones and appear coloured when they 

 have become a good deal stronger. Red lights do not affect the rods at all, and 

 their effect does not appear until they are strong enough to stimulate the cones. 



There is an abnormal type of vision known as TOTAL COLOUR BLINDNESS, 

 which shows a good many points or resemblance to the vision of a dark-adapted 

 eye. People who have this defect see, as far as one can determine, all lights as 

 colourless; rays of different wave-lengths differ to them only in brightness and 

 any two colours can be made to look exactly alike to the colour blind by varying 

 the brightness of one or the other. Two colours which match in their judgment 

 look equally bright also to the normal eye in its condition of dark adaptation, but 

 are different not only in colour but in brightness under ordinary conditions of 

 lighting. Like the dark adapted eye the colour-blind is more sensitive to the 



